The flue gas discharged from thermal power plants is mainly desulfurized by the limestone-gypsum wet method, and a certain amount of waste water will be generated during the desulfurization process. The composition of wastewater quality is complex.
Causes of desulfurization wastewater
1.1 FGD system needs to discharge Cl-
Coal, limestone and process water are the main sources of chloride ions. Generally, the chlorine content in coal is 0.1%-0.01%, the chlorine content in ordinary limestone is about 0.01%, and the chlorine content in process water is 20-200 mg/L. However, the same ion effect leads to an increase in limestone consumption and a decrease in desulfurization efficiency. It is necessary to discharge wastewater, reduce the content of Cl- in the filtrate, and improve the desulfurization efficiency.
1.2 The system needs to discharge ash
The flue gas of the desulfurization system will produce ash, which will not be discharged for a long time, and the dust content will continue to accumulate, resulting in a decrease in the purity of gypsum and the reduction of desulfurization efficiency. Therefore, a certain amount of wastewater must be discharged to improve the purity of gypsum.
1.3 The system needs to discharge inert substances
The purity of gypsum and the normal physical and chemical properties of the system slurry are affected by inert substances. The inert substances in the desulfurizer (limestone) will also accumulate in the system with the recycling of the slurry. Too much accumulation of inert substances will cause the desulfurizer to fail. By discharging a certain amount of wastewater, the purity of gypsum and the normal physical and chemical properties of the system slurry can be improved.
Desulfurization wastewater quality and quantity
The quality and quantity of desulfurization wastewater are affected by factors such as coal type, process water supply, and operation control parameters of the desulfurization system. The water volume of desulfurization wastewater has the characteristics of large fluctuation and discontinuity. The water quality of different thermal power plants will vary. In general, desulfurization wastewater has the characteristics of high salt content, partial acidity, high turbidity, high hardness and many types of pollutants. The pollutants contained in desulfurization wastewater are mainly suspended solids, including fluoride, heavy metals, supersaturated sulfurous acid. The content of Cl in the wastewater is 8000~18,800mg/L, and the desulfurization wastewater is highly corrosive.
Desulfurization wastewater treatment process
The project adopts the conventional FGD desulfurization wastewater treatment process: “triple box treatment + clarification concentration + final neutralization”, the triple box is an integrated box, divided into 3 compartments, including neutralization box, reaction box and flocculation box. , the process flow is as shown below.
Add lime milk to the neutralization box and stir quickly to make the originally acidic wastewater alkaline. An online instrument is installed in the neutralization box to monitor the pH value and control the pH value to be between 8.8 and 9.5. Under this condition, most heavy metals can form Slightly soluble hydroxides precipitate out of the wastewater. Using lime milk as a neutralizer, it can also react with F- to form insoluble CaF2 precipitate, which has the effect of defluorination. At the same time, lime has a flocculation effect on impurities in wastewater.
3.2 Reaction sedimentation
Engineering practice shows that adding organic vulcanizing agent TMT-15 to the reaction box can precipitate heavy metal ions that cannot be precipitated in the form of hydroxides to form sulfides with smaller solubility products. Heavy metal sulfide has low solubility, and has a good treatment effect on cadmium, zinc, mercury, etc. in wastewater, and has small precipitation volume, good chemical stability, and is not easy to dissolve back.
The content of suspended solids in wastewater is high, and flocculation treatment must be carried out when chemical precipitation is carried out. Add polyiron into the flocculation box to make the microflocs gradually form larger flocs, and add polymer polyelectrolyte (PAM) as a coagulant on the outlet pipe of the flocculation box to further increase the flocs and make it easier to settlement.
3.4 Clarification and concentration
The desulfurization wastewater overflows from the flocculation tank into the central guide tube of the clarification and concentration tank, and realizes solid-liquid separation under the action of gravity. The sludge is discharged through the sludge transfer pump.
3.5 Final Neutralization
After the precipitation and separation reaction of desulfurization wastewater is completed, since the pH value is greater than 9, which exceeds the discharge standard, it is necessary to add about 30% industrial hydrochloric acid in the outlet tank for neutralization reaction to neutralize the pH value of the water to 6~9. The outlet tank is equipped with an online monitoring pH value and suspended solids meter. When the suspended solids and pH value in the wastewater meet the discharge standard, the clean water will be discharged up to the standard. If the suspended solids or pH value in the wastewater does not meet the discharge standard, the wastewater will be returned to the neutralization tank for reprocessing until it reaches the standard.
Set up on-line monitoring mud level meter to monitor the sludge height at the bottom of the clarification and concentration tank. When it exceeds the set range, start the sludge transfer pump, and the sludge is pumped into the plate and frame filter press for dehydration, and the mud cake is temporarily stored In the mud bucket, and then loaded out of the truck.
Analysis of existing problems
4.1 Design aspects
(1) The desulfurization wastewater treatment system does not have a wastewater buffer pool. The desulfurization wastewater overflows from the wastewater cyclone directly into the triple tank for treatment. The desulfurization wastewater is discharged intermittently, and the flow rate changes greatly and the suspended solids content is high. There is no wastewater buffer tank at the front end of the wastewater treatment system, and the water quality and water volume cannot be adjusted. It will also cause the clarification and concentration tank to be overloaded in a short time, affecting the quality of the effluent.
(2) The design of the flushing pipeline is not perfect. The desulfurization wastewater system is prone to blockage. The desulfurization wastewater system is not equipped with flushing pipes for the neutralization, reaction and flocculation tanks. After the tank is drained, it cannot be cleaned.
(3) The hydrochloric acid dosing device is designed in an independent cofferdam, but there is no safety eyewash device, which has certain potential safety hazards.
Optimization suggestions and measures
(1) Add a pre-sedimentation tank.
Excessive suspended solids in the desulfurization wastewater will increase the treatment pressure of the triple box, resulting in unstable operation of the agitator in the triple box and poor coagulation effect. A pre-sedimentation tank is set up to effectively remove more than 50% of the suspended solids through gravity precipitation, and the residence time of the pre-sedimentation tank is at least 4 hours; the waste water is then sent to the subsequent process for treatment. In addition, the pre-sedimentation tank can also adjust the water quality and quantity of the incoming water to ensure the stable operation of the triple box system and improve the water effluent effect of the system.
(2) Improve the flushing pipeline.
Set up flushing pipes in the neutralization, reaction and flocculation boxes, and flush the triple box regularly.
(3) The hydrochloric acid dosing device is designed in an independent cofferdam, and a set of safety eyewashes are installed to eliminate potential safety hazards.
(4) Corrosion should be considered for equipment and pipelines in contact with wastewater. The tanks should be lined with rubber or flake resin, and the pipelines should be rubber-lined (plastic) pipes or other anti-corrosion pipes. At the same time, a flushing water pipe and a draining water pipe are set in the pipeline. When the pump stops running, the pump and the pipeline should be flushed in time to remove the waste water. Especially sludge pump, lime milk circulation pump and sludge pipeline, lime milk pipeline.